Recently, flexible pipes are widely used in piping in buildings, etc. for fluids such as gas, water, etc. As shown in FIG. 7, a flexible pipe generally used is produced by corrugating a hoop sheet of stainless steel having a thickness of about 0.2-0.5 mm, and forming a soft vinyl chloride resin layer 4 having a thickness of about 0.5-1 mm on the outer surface of the resultant bendable corrugated metal pipe 2.
When lightening hits near a building, large current of lightening intrudes into the building through conductors such as the ground, the steel frames of the building, electric wires, etc. This phenomenon is called “induced lightning.” Although the corrugated metal pipe is usually grounded or kept at potential extremely close to the ground potential, the induced lightning intruding into the building causes spark discharge, for instance, between the steel frames of the building and the corrugated metal pipe installed nearby, so that apertures of about 1 mm in diameter are likely to be formed in the corrugated metal pipe 2.
Various attempts have been proposed to provide safety measures for flexible pipes to the induced lightning. JP2002-174374A proposes a lightening-resistant pipe comprising a conductive coating layer formed on an outer surface of a flexible metal pipe, the conductive coating layer being conductively connectable to external conductive facilities. JP2003-083483A proposes a lightening-resistant pipe comprising a flexible metal pipe, and an insulating coating layer formed on the outer surface of the flexible metal pipe, the insulating coating layer being provided on its inner surface with longitudinal grooves and a conductive coating layer in contact with the flexible metal pipe, and the conductive coating layer being conductively connectable to external conductive facilities. JP2002-310381A proposes a piping comprising a metal pipe, and a self-fusible, insulating tape made of an ethylene-propylene rubber as a main material and having a thickness of 0.5-0.8 mm, which is wound around the metal pipe to prevent damage by induced lightning. JP2002-315170A proposes a piping comprising a metal pipe, and a self-fusible, insulating tape made of an ethylene-propylene rubber as a main material and wound around the metal pipe, and a grounded, shielding metal layer provided on the self-fusible, insulating tape, to prevent damage by induced lightning. JP2003-083482A proposes a lightening-resistant pipe comprising a flexible metal pipe, an insulating coating layer formed on the outer surface of the flexible metal pipe, and a wire- or strip-shaped conductive member integrally provided on the inner or outer surface of the insulating coating layer, the conductive member being conductively connectable to external conductive facilities.
It has been found, however, that the formation of a conductive coating layer on the outer surface of a corrugated metal pipe as described in JP2002-174374A, JP2003-083483A and JP2003-083482A, and the winding of a self-fusible insulating tape around the metal pipe as described in JP2002-310381A and JP2002-315170A are insufficient as safety measures to the induced lightning.
It is not easy to completely insulate the corrugated metal pipe, and attempts to secure insulation to the spark discharge make the insulating layer too thick, resulting in providing the flexible pipe with reduced flexibility, disadvantageous for piping. It also enlarges the outer diameter of the flexible pipe, thereby needing large joints to be connected.